The suspension polymerization of vinyl chloride is generally carried out at temperatures below 70.degree. C. using organic soluble initiators. Although lauroyl peroxide was earlier the most widely used catalyst, in recent years other low temperature catalysts including azobisisobutyronitrile, diisopropyl peroxydicarbonate, t-butyl peroxypivalate and mixtures thereof, have been adopted. These and other catalysts are described in Pennwalt Corporation, Lucidol Division, Technical Bulletin 30.90, "Free Radical Initiators for the Suspension Polymerization of Vinyl Chloride" (1977).
The choice of initiator is dictated by its half-life and by its influence on the polymerization process and the properties of the poly(vinyl chloride) produced thereby.
The polymerization of vinyl chloride is characterized by a short induction period, followed by a gradually increasing rate of polymerization. During the earlier stages of polymerization, the reaction rate is lower than the maximum so that the capacity of the reactor is not fully utilized. Peroxyesters reduce the induction period and, due to a more constant rate of polymerization, increase reactor productivity. Further, peroxyesters can generally be used at levels below that needed for peroxides and give much less chain branching during polymerization.
Although peroxyesters such as diisopropyl peroxydicarbonate and t-butyl peroxypivalate offer numerous advantages in vinyl chloride polymerization, their disadvantages include the necessity for low temperature shipping and storage and decreased efficiency at elevated temperatures.
The use of peroxyesters having higher decomposition temperatures is not feasible in present poly(vinyl chloride) production facilities due to the higher monomer pressures involved and the low molecular weight and poorer stability of the resultant resins. Nevertheless, the handling advantages of such peroxyesters makes their use extremely attractive.
The use of higher temperature catalysts at lower temperatures is a common practice in polymer technology. Thus, redox systems such as ammonium persulfate-sodium metabisulfite and hydrogen peroxide-ferrous sulfate are used in emulsion polymerization while benzoyl peroxide-dimethylaniline and methyl ethyl ketone peroxide-cobalt naphthenate are used in styrene-unsaturated polyester polymerization.
Reducing agents used in conjunction with monomer-soluble peroxyesters in the polymerization of vinyl chloride include potassium metabisulfite (N. Fischer and C. Lambling, French Pat. No. 2,086,635 (1972), sodium bisulfite (H. Minato, K. Hashimoto, and T. Yasui, Japan. Pat. No. 68 20,300 (1968), sodium bisulfite-cupric chloride (B. K. Shen, U.S. Pat. No. 3,668,194 (1972), sodium dithionite-ferrous sulfate (H. Minato, Japan. Pat. No. 70 04,994 (1970) and trialkyl boron (R. Kato and I. Soematsu, Japan. Pat. No. 5498('65) (1965); A. V. Ryabov, V. A. Dodonov, and Y. A. Ivanova, Tr. Khim. Khim. Tekknol., 1970, 238; Stockholms Superfosfat Fabriks A/B, Brit. Pat. No. 961,254 (1964).
Various sugars including glucose, dextrose, fructose and sorbose have been used as activators in "activated recipes" for the emulsion polymerization of butadiene and styrene. These so-called "sugar-iron-peroxide" recipes contain benzoyl peroxide, cumene hydroperoxide or other hydroperoxides as oxidant, ferric or ferrous salts as reducing agent, a sugar as "activator" and, generally, sodium pyrophosphate as complexing agent (F. A. Bovey, I. M. Kolthoff, A. I. Medalia, and E. J. Meehan, "Emulsion Polymerization", Interscience Publishers, New York, 1955, pp. 85-89 and 374-390). Dextrose has also been used in a cumene-hydroperoxide-ferrous sulfate-sodium pyrophosphate catalyst system for the graft copolymerization of styrene and acrylonitrile onto a polybutadiene latex (W. C. Calvert, U.S. Pat. No. 3,238,275 (1966); T. Sakuma and I. Makamura, Japan Pat. No. 13,635 ('66) (1966).